1. Field of the Invention
The present invention relates to an optical amplifier for optical communications, and more particularly, to a hybrid optical amplifier that couples a Raman fiber amplifier and a semiconductor optical amplifier to perfectly operate in WDM optical transmission systems. The hybrid optical amplifier uses both a gain clamped semiconductor optical amplifier and a Raman fiber amplifier, so that a sufficiently high gain and a sufficiently low noise figure are secured. In addition, the hybrid optical amplifier works perfect in WDM optical networks, in which the number of channels are dynamically changed by an add/drop of channels or the like. The semiconductor optical amplifier and Raman fiber amplifier are packaged in a small size without using an optical fiber, so that the hybrid optical amplifier is convenient to use and low in cost.
2. Description of Related Art
To cope with a recently rapidly increasing optical communication capacity, developments for high-capacity main communication networks and connection of an optical fiber to a subscriber level are actively being performed. In such a system, an optical amplifier is a key device to compensate a loss incurred, for example, by a long distance transmission or/and the insertion of parts.
An erbium-doped fiber amplifier (hereinafter referred to as simply “EDFA”) is usually used as the optical amplifier, and researches to develop new optical amplifiers which are better in size, available bands, and cost are actively pursued. Candidates for such an optical amplifier include a semiconductor optical amplifier (hereinafter referred to as simply “SOA”), a Raman fiber amplifier (RFA), and a hybrid optical amplifier that consists of both an SOA and Raman fiber amplifier.
SOA has advantages that the size is very small, the power consumption is low, the cost can be very low, and an amplification band can be easily adjusted. However, SOA has also disadvantages that the power output is low, the noise figure is worse than typical EDFAS, and the cross-talk between channels is serious.
Researches to use SOA's in a metro-WDM system which is in great demand are restrictively performed. However, it is not widely accepted because it has difficulties to use due to a relatively high noise figure and an low output power in order to avoid significant cross-talks between channels.
Meanwhile, RFA has an advantage that it is easy to adjust an amplification band just by varying a pump laser wavelength using an transmission optical fiber as a gain medium. In particular, when it is coupled to the existing EDFA, a longer distance transmission is possible since a gain is increased and an effective noise figure is lowered. However, when RFA is not used together with other amplifiers such as an EDFA, but is used alone, its use is actually very difficult because a pump light of about 1 W should be inseterted into a single mode optical fiber in order to get a sufficient gain, i.e., more than 20 dB.
A hybrid type optical amplifier has been recently developed that couples an SOA and an RFA in order to solve problems of the previous amplifiers. FIG. 2 is a block diagram illustrating the hybrid type optical amplifier coupling the SOA and the RFA.
As shown in FIG. 2, a signal light incident through a transmission optical fiber 110 is amplified due to a simulated Raman scattering incurred in a transmission optical fiber by pumping laser diodes (LD) 121 having a high output proceeding in a reverse direction through a wavelength division multiplexer 122. The signal amplified by the reverse pumping Raman fiber amplifier passes through a first isolator 130 and is incident to the SOA 140 and then proceeds tens of kilometers. Thereafter, the same operation is repeated in the next step.
Since the signal incident to the SOA 140 is amplified beforehand with an addition of negligible noise the magnitude of noise added in the SOA 140 is relatively small, whereby an effective noise figure is lowered. In such a way, the gain of the SOA 140 is increased and the noise figure is lowered, so that the hybrid type optical amplifier can operate as an optical amplifier of a high performance. In this case, since the SOA 140 is made of a semiconductor chip and Raman pumping laser diode LD 122 is made of another semiconductor chip, the cost of the system can be sufficiently low.
However, the conventional art described above is hard to properly perform a WDM signal amplification. This is because when several channels of different wavelengths are amplified, since gain characteristics are homogeneous and gain dynamics are fast about 1 ns, a cross-talk phenomenon that a total gain value is varied according to an input signal variation of one channel, so that gain values of other channels are varied. As a result, a sufficiently small input signal in only a region where the gain saturation is not serious should be used, in order to avoid the cross-talk problem. This system cannot be used in WDM networks in which dynamic add/drops are frequent.
On the other hand, to remove a cross-talk from the conventional SOAs of the previous art, as shown in FIG. 3, a type of gain-clamped SOA (GCSOA) 220 having a clamped gain regardless of a power of an input optical signal are reported. Another type of GCSOA clamping a gain by providing a feedback in a direction of a light axis and a GCSOA, known as LOA, clamping a gain by providing a feedback in a perpendicular direction to a light axis has been commercially available very recently. The LOA is excellent in gain clamping characteristic and shows no relaxation oscillation to 20 GHz. In this case, since there is no cross-talk and the gain per channel is not varied even though there is an add/drop of channels, the GCSOAs can be used in the WDM networks of FIG. 1. The GCSOA (LOA) is much more excellent in gain clamping characteristic than the gain clamped EDFA.
However, like the conventional SOA, the GCSOA, including LOA, is also low in output and is high in noise figure and so can be used only for transmissions of short distance.
Furthermore, the hybrid type optical amplifier of FIG. 2 uses the SOA pigtailed with an optica fiber and a laser diode also pigtailed with an optica fiber. In this case, there is a limitation in lowering a cost because it is a difficult to package the semiconductor chip into an optical fiber and most of the device cost comes from the packaging. Besides, since respective parts are coupled by an optical fiber which is not short, there is a problem in that a total size of the hybrid optical amplifier becomes large.